In modern industrial control, processor based devices such as programmable logic controllers (PLC's) perform a majority of control operations on a production line. The processor based device can be coupled to a data communication network by a network interface module so that the processor based device can communicate with other processor based devices also coupled to the data communication network. However, as with any machine, such processor based devices and associated hardware and software are subject to failure. In addition it is often necessary to shut down the processor based device to perform preventative maintenance. Often this failure or planned shut down will result in a complete shutdown of the production line.
To minimize this problem, redundant processor systems have been developed wherein a single processor based device is replaced by a pair of processor based devices. One of the processor based devices operates as a primary processor based device and the other operates as a backup processor based device. In the event of a failure of the primary processor based device, the backup processor based device takes over control.
In order to insure that messages are sent to the primary processor based device, both the primary and backup processor based devices were assigned the same data addresses for receiving messages. However only the primary processor based device would monitor the data communications network. The back up processor based device was dormant, and therefore effectively prevented from communicating with other processor based devices on the data communications network.
Accordingly because the back up processor based device was shut off from the data communications network, other devices could not communicate with it, such as to perform such functions as to load new programs, to perform diagnostics or to check status.
The present invention is provided to solve these and other problems.